A new study shows that up to 25 percent of women who have harmless breast tumors undergo invasive cancer treatments anyway, because mammograms don't differentiate between agressive cancers and those that would never cause symptoms.

It might sound weird that some cancers would never develop into full-blown disease—but harmless breast cancers apparently exist. Doctors usually recommend that women begin routine mammograms in their 40s or 50s. And it stands to reason that: better safe than sorry, right? But how early and often they should be done has become controversial in recent years because of the overdiagnosis problem. They can lead to not just an unnecessary (but natural) emotional freak out but also unwarranted chemotherapy and mastectomies.

Norwegian scientists looked at data from nearly 40,000 women with invasive breast cancer in Norway, of whom 7,793 had been diagnosed as part of a screening program that recommended mammograms. They found that between 15 to 25 percent, or between 1,169 and 1,948 women, were overdiagnosed. Based on those numbers, they estimate that for every 2,500 women screened, between 6 and 10 women will be overdiagnosed, treated with surgery, radiation therapy, and possibly chemotherapy without any benefit, and one death would be prevented. The work is published in the April 3 issue of the Annals of Internal Medicine.

Lead author Mette Kalager, a visiting scientists at the Harvard School of Public Health and a researcher at the Telemark Hospital in Norway had this to say in a statement:

Mammography might not be appropriate for use in breast cancer screening because it cannot distinguish between progressive and non-progressive cancer. Radiologists have been trained to find even the smallest of tumors in a bid to detect as many cancers as possible to be able to cure breast cancer. However, the present study adds to the increasing body of evidence that this practice has caused a problem for women-diagnosis of breast cancer that wouldn't cause symptoms or death.

They also found that overall, mammography did not reduce late-stage breast cancer cases, which the researchers say would be expected if you figure early detection prevents late-stage disease.

But then there's the study from 2010 that found women's lives could be saved by starting mammograms at 40. What's a breast-owner to do with so much conflicting information? I suppose it's up to us to get as informed as possible and decide for ourselves. And to somehow digest the fact that mammography could save our lives and it could also give us unnecessary mental distress, biopsies, surgeries, chemotherapy and hormone treatments. [Annals of Internal Medicine]

Update: The headline of this post originally read: Mammogram False Positives Lead to Needless Chemotherapy, Mastectomies, and has been updated to reflect the fact that the study was not assessing false positives but overdiagnosis of breast cancers that would have never developed into a health problem for the patient.